The present invention relates to a demodulator with an improved local oscillator, for use mainly in the field of cellular telephony, in particular in the Global System for Mobile communications (GSM). In the field of cellular telephony, recognition protocols are instituted at the time a mobile telephone enters service in a network. These protocols impose technical constraints which are difficult for the equipment of the mobile telephone to satisfy. The object of the present invention is to provide an effective solution to a problem which occurs at the time of entry into service and which, in the prior art, can be solved only by choosing perfect local oscillators whose development cost is excessive.
When a mobile telephone enters service in a cellular network, it enters into contact with a base station. Since the mobile telephone does not know in which cell of the network it is located, an entry into service routine includes seeking the most powerful beacon signal at the location of the mobile telephone. The beacon signal referred to is broadcast by the base station of a cell and includes various types of signal needed for new mobile telephones to log on in the cell and to maintain the surveillance of mobile telephones already present. The beacon signal is generally transmitted at a fixed beacon frequency Fi which can vary from one cell to another. In some cases some adjacent cells can share the same beacon frequency. In all cases, the signal transmitted at the beacon frequency is transmitted at a higher power than call signals exchanged between the base station of the cell and a mobile telephone, even the telephone in the cell at the greatest distance from the base station. Also, the beacon signal is broadcast continuously. Although it is divided into frames, and within those frames into time slots whose meaning changes from one slot to another, transmission is constant and at the same power level.
Without going into too much detail, the beacon signal is formed by a pattern repeated at the end of the aforementioned particular number of frames, for example 51 frames in the GSM. In this case the duration of a time slot is 577 microseconds and the duration of a frame is 4.615 milliseconds, because a frame includes eight time slots. The duration of a pattern, which includes groups of ten or eleven frames, is of the order of 235 milliseconds.
A first group of frames of the pattern is different from the next group. It includes FCH, SCH, BCCH and CCCH signals in each of its ten frames. The FCH (frequency control channel) signals correspond to the transmission of a carrier Fi (the control carrier) modulated by a pure sinewave offset by 67.7 kHz from the center of the channel. With this modulation frequency, in GSM coding with a modulation constellation with twelve points, code values I and Q with successive values (1,0), (0,1), (xe2x88x921,0) and (0,xe2x88x921) are transmitted and repeated. The modulated carrier is transmitted throughout a time slot.
A mobile telephone which has just entered service can therefore first determine which carrier it receives at the highest power. In a standard 35 MHz band allocated under the GSM 900 MHz standard (other bands are allocated under the DCS, PCS, and even UMTS standards), the mobile telephone determines which carrier at a beacon frequency it is receiving the best. It does this by scanning the band and applying a simple detection process, for example directly at the output of a receiver circuit, and even prior to demodulation. This scanning includes monitoring channels of standard width, which is 200 kHz in the GSM. Although the carrier frequencies dedicated to transmitting call signals are subject to frequency hopping from one time slot to another, the beacon frequency Fi is constant. It is therefore sufficient for a mobile telephone to monitor all the bands in succession to determine which one it is receiving the best.
When a beacon frequency Fi has been identified, the mobile telephone determines in which time slot it receives an FCH signal modulating the carrier Fi with a pure sinewave at 67.7 kHz. The problems addressed by the invention arise more particularly in this process. This is because, given the imposed channel width of 200 kHz, the demodulators in the demodulator circuits normally have a channel filter at their output. The bandwidth of a low-frequency channel filter runs from xe2x88x92100 kHz to 100 kHz. Demodulation transposes the modulated signal by mixing the received signal with a signal produced by a local oscillator at the carrier frequency Fi. However, demodulation is not perfect, because of imperfections of the local oscillator. If the local oscillator produces a signal at a frequency Fi+xcex5 instead of producing a signal at a frequency Fi, the demodulated output signal will have a frequency equal to 67.7 kHz+xcex5.
Of course, if xcex5 is small there is no problem. In particular, when a mobile telephone is new, i.e. during its manufacture, the local oscillator can be set so that xcex5 has a value that is small or zero. In practice, referred to the beacon frequency Fi, the acceptable offset xcex5 must be less than 23.5 parts per million (ppm) in the 900 MHz GSM band, less than 12.1 ppm in the 1800 MHz DCS band and less than 11.4 ppm in the 2100 MHz PCS band. This avoids demodulation problems due to the channel filter. Also, correction circuits are provided in a mobile telephone to measure the offset xcex5 and to modify the values of the demodulated signals accordingly (and not to modify the demodulation frequency Fi+xcex5). In the demodulation systems used it is not really possible to modify the center demodulation frequency produced by the local oscillator, for example by means of a control loop. Why the architecture of these local oscillators rules out such adjustment of the demodulation frequency is explained below. In contrast, if the offset xcex5 is small, and in particular if it is within the limits indicated above, it is possible to correct the demodulated signals so that on decoding they produce a value which allows for the measured offset xcex5. The offset xcex5 is measured at the time of reception of the FCH signals.
Unfortunately the local oscillator frequency can drift due to aging of the equipment, and in some cases because of the conditions of use of the mobile telephone, such as temperature and voltage. The drift can be sufficient for the channel filter to interfere with the reception and demodulation of the signal at the frequency of 67.7 kHz. This is because there is a distinct slope, rather than a sharp cut-off, at the edge of the band of the channel filter, at around 100 kHz. In practice, received signals begin to be attenuated well short of a limit frequency of 100 kHz. Note that, for reception of call signals, this progressive cut-off (which has the advantage of simpler implementation of the channel filter and a lower penalty in terms of phase rotation) is not a problem because the quantity of information transmitted per hertz at the edge of the band is small. Most of the information is contained in the demodulated low frequencies.
In accordance with the invention, to remedy these drawbacks without having to adjust the filter, the local oscillator frequency is shifted upwards so that under optimum adjustment conditions, and in particular when it is brand new, i.e. immediately after manufacture, it produces a signal at a beacon frequency slightly higher than the expected standard frequency Fi. This being the case, in the demodulation process, a signal at a frequency Fi+xcex5xe2x80x2 is subtracted from a received modulated carrier signal instead of a signal at a frequency Fi. Accordingly, and especially in the case of reception of the FCH signals, the demodulated signal will no longer be a signal at a frequency of 67.7 kHz but a signal at a frequency of 67.7 kHzxe2x88x92xcex5xe2x80x2. Consequently, if the local oscillator drifts because of aging, there is a margin for drift equal to xcex5xe2x80x2 even before reaching the beginning of the tolerance range imposed by the channel filter. Consequently, even after several years"" use, detection of the FCH signals is improved and the telephone logs onto a base station more efficiently.
In practice, the absolute value of the shift xcex5xe2x80x2 is low. Its contribution can easily be compensated by the correction circuits already included in the receiver circuits and which neutralize the prior art shift xcex5. The shift xcex5xe2x80x2 of the local oscillator is obviously upward so that the demodulated signal FCH is pulled towards low frequencies, to become a signal at less than 67.7 kHz, rather than towards high frequencies where, on its frequency becoming much greater than 67.7 kHz, it could reach the cut-off band of the channel filter, where its attenuation would purely and simply prevent it from being detected correctly.
The invention therefore provides a mobile telephone including a demodulator circuit including a local oscillator and a mixer with a local oscillator input and a demodulation input, the demodulation input receiving a signal modulating a carrier at a frequency Fi, wherein the output of the local oscillator is off-tuned upwards relative to the frequency Figure.